The present invention relates to recordable (once-writable) and rewritable information-recording media, and more particularly, to optical discs.
Recently, the densification of such optical discs, which are used as recording devices for computers, or package media for music and image information, has been advanced. In order to have precise, high-speed access to such high-density tracks, it is necessary to provide preformatted signals having address information to a recording surface of the optical disc, to and from which surface information is recorded and reproduced. Further, information that one does not want to erase can also be formed in advance as preformatted signals on the disc. Portions other than the preformatted portions are a recording area in which guide grooves referred to as simply xe2x80x98groovesxe2x80x99 are formed. In the groove area, namely, in the recording area, grooves that are concave portions and lands that are not concave portions exist.
FIGS. 5A and 5B show a disc of related art, and are a top view and a perspective view of the disc, respectively. In FIG. 5B, a cross section of the disc is also shown. Reference symbols xe2x80x98Gxe2x80x99 and xe2x80x98Lxe2x80x99 indicate a groove and a land, respectively. Reference symbol xe2x80x98PPxe2x80x99 indicates a prepit. Light 3 (typically, a laser beam) is condensed by a lens 2 and then incident through a substrate 1. Comparing the land with the groove, the groove is nearer to the lens 2.
A recording layer of a material represented by a magneto-optical material, a phase-change material or an organic dye material is formed (not shown in the figures) on the lands, grooves, and prepits. In the figures, a recording mark M is written in the groove. This is because better signals in quality are obtained when the information is written in the grooves than when written on the lands.
When recording/reproducing information onto/from the optical disc on which the guide grooves are formed as described above, tracking servo control is implemented by a push-pull method using a servo control signal called the xe2x80x98push-pull signalxe2x80x99 resulting from the diffraction of light at the grooves and prepits. A light beam (xe2x80x98beam spotxe2x80x99) condensed by the lens 2 tracks on the grooves and prepits.
Generally, the push-pull signal in the prepit area has a smaller amplitude than the push-pull signal in the groove area. The reason for that is as follows. The structure of the prepit area in which concave portions are spaced from each other is equivalent to the structure of a broken, or discontinuous groove. Therefore, in the prepit area, due to the discontinuity of the concave portions, the amount of diffraction of light is reduced accordingly on the average. This is why that the amplitude of the push-pull signal is smaller in the prepit area than in the groove area.
Accordingly, if the gain of the tracking servo control is adjusted so that it is suitable for the groove area, the amplitude of the push-pull signal in the prepit area becomes too small. Therefore, the gain of the tracking servo control is relatively small for the prepit area, and the control precision deteriorates. In contrast, if the gain of the tracking servo control is adapted to the prepit area, the amplitude of the push-pull signal in the groove area becomes excessively large. Thus, disadvantageously, oscillation occurs in the tracking servo system.
The present invention has been made in order to solve the above problems, and the object of the present invention is to provide a highly reliable optical recording medium with a prepit area wherein recording is implemented at least at grooves, and push-pull signals obtained in a groove area (recording area) and a prepit area have similar magnitudes so that precise tracking can be implemented without deterioration of control precision nor occurrence of oscillation of the tracking servo control system, whereby complication of the circuitry of a recording/reproducing device and the increase of the costs of such a device can be avoided.
In order to accomplish the above object, according to an aspect of the present invention, there is provided an optical recording medium, comprising a substrate having a recording area, composed at least of lands and grooves, and a prepit area in which information has been recorded beforehand in the form of prepits, the recording area being recordable at least at the grooves among the lands and grooves, the optical recording medium being constructed such that a width of each groove, Wg, a width of each land, Wl, and a width of each prepit, Wp, satisfy the following relationships of:
either Wl less than Wg, and Wp less than Wg, or Wl greater than Wg, and Wp greater than Wg.
With this arrangement, the amplitudes of the push-pull signals from the recording area (groove area) and the prepit area can be made equivalent to each other or little different from each other. Thus, it becomes possible to achieve accurate tracking servo control. Accordingly, there is an effect that it is possible to prevent the complication of the circuitry and the cost increase of a recording/reproducing apparatus for the optical recording medium.
According to another aspect of the present invention, there is provided an optical recording medium, comprising a substrate having a recording area, composed at least of lands and grooves, and a prepit area in which information has been recorded beforehand in the form of prepits, the recording area being recordable at least at the grooves among the lands and grooves,
wherein a sum of lengths of pit-presence portions in the prepit area is larger than a sum of lengths of pit-absence portions in the prepit area.
The inventor of this invention has found that by increasing duty of prepits (also referred to as simply xe2x80x98pitsxe2x80x99) which is calculated from (the sum of lengths of the pit-presence portions in the prepit area)/(the sum of lengths of the pit-presence portions and pit-absence portions in the prepit area), it is possible to increase the amplitude of the push-pull signal from the prepit area. The optical recording medium with this arrangement has duty of prepits of more than 50%. Thus, even if the prepit width is same as the groove width so that there is a difference in amplitude of the push-pull signal between the groove area and the prepit area, the difference can be decreased. As a result, it is possible to achieve accurate tracking servo control. Furthermore, because the same width can be used for the grooves and the prepits for the above reason, the optical recording medium can be simply fabricated.
The above characteristic features of the first and second aspects of the present invention may be combined so that not only the widths of the grooves and prepits but also the duty of prepits are changed. In this case, freedom is increased in designing the optical recording medium such that there is no difference or little difference between the amplitudes of the push-pull signals from the groove area and the prepit area.
Other objects, features and advantages of the present invention will be obvious from the following description.